Fix the PRACH FFT size for 7.2 interface

We assumed that PRACH FFT size is equivalent to PxSCH FFT size.

However, the parameter ru_config->fftSize is used for the C-plane Section Type 3
which is dedicated for PRACH and mixed numerology.
Based on the PRACH format, short or long, the PRACH values 256 or 1024
shall be filled.

Co-authored-by: knopp <raymond.knopp@eurecom.fr>

executables/nr-ru.c

@@ -896,6 +896,7 @@ static void fill_split7_2_config(split7_config_t *split7, const nfapi_nr_config_
     }
   }
 
+  split7->prach_fftSize = prach_config->prach_sequence_length.value == 0 ? 10 : 8; // need to handle 5kHz cases better than this
   split7->fftSize = log2(fp->ofdm_symbol_size);
 
   // M-plane related parameters